1. Technical Field
The present invention relates to an apparatus and a method for distance measurement to a target using a laser beam.
2. Background Art
An apparatus for laser distance measurement measures a distance to a desired target based on a length of time between a time of emitting the laser beam to the target and a time of receiving the laser beam reflected by the target (hereinafter, “turnaround time”).
For example, Japanese Patent Application Laid-Open No. 1999-287859 discloses a laser distance measuring apparatus, which includes a laser element that emits the laser beam to a target, a laser driving module which drives the laser element, and a detecting module that receives the laser beam reflected by the target and detects a serrate wave signal mixed with the emitted laser beam mixed in the reflected laser beam. The detecting module includes a band-pass filter which allows a frequency component within a predetermined frequency band of the serrate wave signal to pass, and a signal level measuring module for measuring level of a signal which passes the band-pass filter.
Japanese Patent Application Laid-Open No. 2006-030181 discloses a method for measuring a distance by a chromatic dispersion focusing element which focusing a laser beam at a point on a target face.
A laser distance measuring apparatus above mentioned is applied to various fields. In particular, the distance measuring apparatus may be applied to space rendezvous in which a transporting spacecraft (e.g. the Space Shuttle) docks with either an artificial satellite or a space station. Such distance measuring apparatus requires high accuracy, high reliability and wide dynamic range. Similarly, the distance measuring apparatus may be applied to a guided landing of a helicopter. In such a field, high accuracy, high reliability and wide dynamic range are also required.
The transporting space vehicle includes the distance measuring apparatus in order to dock with either the artificial satellite or the space station. Each of the artificial satellite and the space station includes a marker used for a target of the laser beam.
The distance measuring apparatus emits a pulsed laser beam to the target. A scanner scans the target in a two-dimensional manner by the pulsed laser beam. Then, the distance measuring apparatus measures the turnaround time of the pulsed laser beam and data on an emitting angle of the pulsed laser beam. Information on a distance to the target is obtained on the basis of the turnaround time, and information on a target position is obtained on the basis of the data on the emitting angle.
The receiving light level (an amount of a receiving light) of the laser beam fluctuates within a level which is 106 or 107 times higher than a predetermined initial value according to a distance to the target. For this reason, an amplifier in a light detector and a light receiver may be saturated, and an electric signal which is converted from a light signal may be distorted consequently. The signal distortion degrades precision of distance measurement.
Thus, a gain of the light detector is changed according to the receiving light level, or a variable wavelength filter is installed in an optical path. Accordingly, degradation of precision of distance measurement due to the fluctuation of the receiving light level is prevented.
However, changing the gain of the light detector cannot compensate the signal distortion completely, and may cause increasing noises. The signal distortion and the noises degrade precision of distance measurement. For this reason, when the gain of the light detector is changed, fluctuation of the receiving light level to be allowed is limited in the range of only a level which is 104 times higher than the predetermined initial value.
Meanwhile, optically changing the receiving light level, the variable filter is not so influential to the precision of the distance measurement. However, in the existing variable wavelength filter, the filter is mechanically driven to change transparence. Thus, it is difficult for the variable wavelength filter to guarantee reliability requested for a space apparatus.
According to the variable wavelength filer, the receiving light level fluctuation to be allowed is limited in the range of only a level which is 104 times higher than the predetermined initial value. Accordingly, in order to treat fluctuation of a receiving light level which is 106 times higher than the predetermined initial value, it is necessary for the variable wavelength filter to be used together with another apparatus or method.